text: Tritium is a radioactive isotope with a half-life of 12.355 years. Its main decay product is helium-3, which is among the nuclides with the largest cross-section for neutron capture. Therefore, periodically the weapon must have its helium waste flushed out and its tritium supply recharged. This is because any helium-3 in the weapons tritium supply would act as a poison during the weapons detonation, absorbing neutrons meant to collide with the nuclei of its fission fuel.
  Tritium is relatively expensive to produce because each triton - the tritium nucleus - produced requires production of at least one free neutron which is used to bombard a feedstock material (lithium-6, deuterium, or helium-3). Actually, because of losses and inefficiencies, the number of free neutrons needed is closer to two for each triton produced (and tritium begins decaying immediately, so there are losses during collection, storage, and transport from the production facility to the weapons in the field.) The production of free neutrons demands the operation of either a breeder reactor or a particle accelerator (with a spallation target) dedicated to the tritium production facility.